Metallic composite phase-change materials and methods of using the same

ABSTRACT

Metallic composite phase-change materials and methods of using are disclosed. In some embodiments, a thermal energy storage module is provide that included one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application No. 61/717,068, filed on Oct. 22, 2012, which is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with Government Support under Contract Number DE-AR0000181 awarded by the Advanced Research Projects Agency—Energy, Department of Energy. The Government has certain rights in the invention.

FIELD

The embodiments disclosed herein relate to composite phase-change alloys and methods of their synthesis and use.

BACKGROUND

Concentrated solar-thermal power (CSP) generation is a promising clean energy technology for the future. However, solar energy harvesting suffers from facts such as unpredictable interruptions due to weather fluctuations and lack of sun light in night time. To reduce weather dependence of CSP and provide uninterrupted electricity 24 hours a day, thermal energy storage (TES) can be employed. The use of TES for CSP can significantly increase the CSP capacity factor from about 30% to greater than about 60% which in turn can reduce the levelized cost of electricity produced. TES can also be used in addition to traditional nuclear power plants to provide peaking power by storing part of the thermal energy from the nuclear reactor to subsequently run a thermodynamic cycle, such as the Brayton cycle, for power generation. Such application will reduce the dependence on fossil fuel peaking power plants and consequently reduce the CO₂ emission.

In order to achieve higher efficiency, both CSP and the fourth-generation nuclear power plants operate at high temperatures. For example, the dish-Stirling engine CSP system has a gas temperature of over 700° C., and the dish-Brayton application has a turbine inlet temperature of about 850° C. These dish-engine systems have demonstrated the highest solar-to-electricity efficiency of 29.4%. In another study, reported a nuclear power plant design based on supercritical CO₂ with a predicted plant efficiency of 51%. In such a design, CO₂ is heated from 488.9° C. to 879.4° C. through an intermediate heat exchanger between the nuclear reactor fluids and CO₂. Existing research focus of TES has been on systems operating at below 500° C. Hence, there is an urgent demand on the development of TES materials operating in the high temperature range to enable continuous operation of CSP and peaking power capability of nuclear power plants which can result in a great increase in the share of these alternative energy systems.

SUMMARY

Metallic composite phase-change materials (PCMs) and methods of their use and synthesis are disclosed.

In some aspects, there is provided a thermal energy storage module that includes one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg. In some embodiments, the one or more phase change alloys have a formula of Aluminum (Al)-Silicon (Si)—X, wherein X is one or more elements selected from the group consisting of boron (B), carbon (C), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), germanium (Ge), molybdenum (Mo), nobium (Nb), nickel (Ni), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V), and zirconium (Zr). In some embodiments, the one or more phase change alloys have a formula of Al—Si—Fe, where the concentration of Al is between about 40 atom percent and about 50 atom percent, the concentration of Si is between about 35 atom percent and about 45 atom percent and the concentration of Fe is between about 10 atom percent and about 20 atom percent. In some embodiments, a phase change alloy has a formula Al—Si—Fe, where the concentration of Al is about 45 atom percent, the concentration of Si is about 40 atom percent, and the concentration of Fe is about 15 atom percent. In some embodiments, a phase change alloy is Al—Si—B, where the concentration of B is between about 8 atom percent and about 28 atom percent.

In some aspects, there is provided a method of storing heat that includes providing one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg; charging the one or more phase change alloys by allowing heat transfer into the phase change alloy; and storing heat in the one or more phase change alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained with reference to the attached drawings, wherein like structures are referred to by like numerals throughout the several views. The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the presently disclosed embodiments.

FIG. 1, FIG. 2, FIG. 3 and FIG. 4 provide thermal characteristics data for various elements, including heat of fusion in FIG. 1, thermal conductivity in FIG. 2, melting point in FIG. 3 and vapor pressure in FIG. 4.

FIG. 5 and FIG. 6 provide a list of possible eutectic alloys that may be used for the base alloy in the phase change materials (PCMs) of the presnet disclosure.

FIG. 7 provides melting temperature and latent heat data for various alloys.

FIG. 8 provides latent heat data for various alloys with melting temperature between about 800° C. and 900° C.

FIG. 9 presents thermal characteristics of Al-12Si alloy.

FIG. 10 presents latent heat and melting temperature data for Al-12Si alloy.

FIG. 11 presents thermal diffusivity data for Al-12Si alloy.

FIG. 12A, FIG. 12B, FIG. 12C and FIG. 12D present latent heat data for various eutectic and hyper-eutectic Al—Si alloys.

FIG. 13A and FIG. 13B provide latent heat data for pure aluminum and germanium, respectively.

FIG. 14A and FIG. 14B present latent heat data for a Al-12Si-4B alloy.

FIG. 15 presents a phase diagram for a near-eutectic Al—Si—Fe alloy.

FIG. 16 presents a schematic diagram of thermal energy storage and release by a composite phase change material of the present disclosure.

FIG. 17A presents a SEM image of Al-12Si alloy prepared with hot press and results of latent heat test of such alloy.

FIG. 17B presents results of latent heat test of Al-12Si alloy prepared with hot press.

FIG. 18A, FIG. 18B, FIG. 18C, FIG. 18D present SEM images of Al-20Si (by weight percent) alloy prepared with hot press.

FIG. 19A, FIG. 19B, and FIG. 19C are SEM images of Al powder, Si powder, and Fe powder, respectively.

FIG. 19D is an SEM image of a near-eutectic Al—Si—Fe alloy synthesized using DC hot-press method.

FIG. 20A and FIG. 20B are SEM images of a surface of a ternary near-eutectic Al—Si—Fe alloy synthesized by hot press at 700° C.

FIG. 20C and FIG. 20D are SEM images of a surface of a ternary near-eutectic Al—Si—Fe alloy synthesized by hot press at 500° C.

FIG. 21A and FIG. 21B present differential scanning calorimeter (DSC) experimental results for a ternary near-eutectic Al—Si—Fe alloy synthesized by hot-press at 1000° C., with applied pressure around 110 MPa.

FIG. 21C and FIG. 21D present DSC experimental results for a ternary near-eutectic Al—Si—Fe alloy synthesized by hot-press at 700° C., with applied pressure around 94 MPa.

FIG. 21E and FIG. 21F present DSC experimental results for a ternary near-eutectic Al—Si—Fe alloy synthesized by hot-press at 500° C., with applied pressure around 94 MPa.

FIG. 22 illustrates formation of a phase diagram with a eutectic point.

FIG. 23 illustrates a phase diagram for a Cu—Si alloy.

FIG. 24 illustrates a phase diagram for a Pd—Si alloy.

FIG. 25 illustrates a phase diagram for an Al—Fe—Si alloy

While the above-identified drawings set forth presently disclosed embodiments, other embodiments are also contemplated, as noted in the discussion. This disclosure presents illustrative embodiments by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of the presently disclosed embodiments.

DETAILED DESCRIPTION

Metallic composite phase-change materials (PCMs) and methods of their synthesis and use are disclosed. In some embodiments, the PCMs disclosed herein can be used as part of a thermal energy storage module for high-temperature thermal storage and release. In some embodiments, PCMs of the present disclosure can have variable phase transition temperatures to achieve maximum energy efficiency for thermal energy storage. The PCMs of the present disclosure can have one or more of the following characteristics: phase change temperature variable between about 400° C. and about 1200° C. and, in some embodiments, between about 600° C. and about 900° C., large latent heat (more than about 200 kJ/kg and, in some embodiments, more than about 500 kJ/kg), high thermal conductivity values, high stability, long lifetime, low vapor pressure, and low-cost. FIG. 1, FIG. 2, FIG. 3 and FIG. 4 provide thermal characteristics data for various elements, including heat of fusion, thermal conductivity, melting points and vapor pressure. In some embodiments, PCMs of the present disclosure include a Al—Si based alloy and one or more additional elements. In some embodiments, the metallic composites phase-change material is a near-eutectic Al—Si—Fe alloy. In some embodiments, the metallic composites phase-change material is a ternary near-eutectic alloy 45Al-40Si-15Fe. In some embodiments, the phase-change material of the present disclosure is a Al—Si—B alloy. In some embodiments, PCMs of the present disclosure are synthesized by uniformly mixing powders of base alloy elements and one or more additional elements, and hot pressing the resulting mixture.

Latent heat storage is attractive because materials undergoing phase change have high energy storage density which can lead to more compact systems, such as, for example, solar home systems. Latent heat storage can be achieved with PCMs. Although liquid-gas phase transition can usually store the largest amount of energy, the significant change of volume may be problematic in practical engineering applications. Solid-liquid phase transition, however, does not involve large volume changes and it can still store a large amount of energy.

Solid-liquid PCMs divided into the categories of organic, inorganic (non-metal) and eutectic (metal/alloy) PCMs. The most widely used organic PCM is paraffin wax, which has relatively high latent heat (140-280 KJ/kg), good melting characteristics, and it is non-toxic, chemically stable, and relatively cheap. Although organic PCMs are popular, they generally have very low thermal conductivity (<0.6 W/mK) which may limit its output power density and the exergetic efficiency. On the other hand, organic PCMs also suffer from their relatively large volume change during phase transition, and they usually have low melting temperatures (<100° C.).

Inorganic (non-metal) PCMs like salt hydrates are also widely used for TES. Inorganic PCMs have similar latent heat per unit mass as that of the organic PCMs. Due to their higher density, the volumetric latent heat of inorganic PCMs is about twice as much as that of organic PCMs. However, these PCMs may fall short of the basic requirements of PCMs for TES application in concentrated solar power (CSP) systems.

Eutectics (metal/alloy) PCMs, generally referred to as mixtures of one or more metals plus inorganic compositions, melt and freeze without segregation since they freeze to an intimate mixture of crystals, leaving little opportunity for the components to separate. In some embodiments, the PCM of the present disclosure is a near-eutectic alloy, including a base alloy and one or more additional alloying elements added to arrive at desired thermal properties, that is, melting temperature and latent heat, among others. FIG. 5 and FIG. 6 list possible eutectic alloys that may be used for the base alloy in the PCMs of the present disclosure.

In some embodiments, the base alloy is a silicon-based alloy, such as aluminum-silicon (Al—Si), germanium silicon (Ge—Si), ferum-silicon (Fe—Si), manganese-silicon (Mn—Si), berillium-silicon (Be—Si), and similar alloys, as shown, for example, in FIG. 7 and FIG. 8.

In some embodiments, the base alloy may be a aluminum-silicon (Al—Si) alloy, where the concentration of Al may vary between about 0 atom percent (at. %) to about 100 at. % and the concentration of Si may thus also vary between about 0 at. % to about 100 at. %. FIG. 9 presents thermal characteristics of Al-12Si alloy. FIG. 10 presents latent heat and melting temperature data for Al-12Si alloy. The heat of fusion of Al is about 397 kJj/kg, with a melting temperature of about 660° C., and the heat of fusion of Si is about 1800 kJ/kg, with a melting temperature of about 1410° C. FIG. 11 presents thermal diffusivity data for Al-12Si alloy. To calculate thermal conductivity of Al-12Si alloy, C_(p) from literature can be used, where K=45*1*2.7=122 W/mK at 500 C. FIG. 12A, FIG. 12B, FIG. 12C and FIG. 12D present latent heat data for various eutectic and hyper-eutectic Al—Si alloys.

In some embodiments, adding one or more additional elements to the base alloy may result in improvement of one or more thermal properties of the resulting alloy. For example, the addition of one or more additional alloying elements to the base alloy may result in the resulting material having a variable melting temperature between about 400 and about 1200° C., latent heat of greater than 200 kJ/kg, or both.

The alloying elements may be selected based on their thermal properties, as well as the desired thermal properties of the final PCM alloy. By way of a non-limiting example, FIG. 13A and FIG. 13B provide melting temperature and latent heat data for pure aluminum and pure germanium, which can be used as additional alloying element. The one or more alloying elements added to the base alloy can be selected from B boron (B), carbon (C), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), germanium (Ge), molybdenum (Mo), nobium (Nb), nickel (Ni), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V), and zirconium (Zr).

In some embodiments, the PCM of the present disclosure is a Al—Si—B alloy. In some embodiments, the concentration of Boron may range between about 8 at. % and about 28 at. %. By way of non-limiting example, FIG. 14A and FIG. 14B present latent heat data for Al-12Si-4B. DSC results show that the latent heat of eutectic Al-12Si alloy might be increased from about 555 kJ/kg to about 628 kJ/kg (+13%) by adding B powder at about 4 wt % (or 10 at %).

In some embodiments, the PCMs of the present disclosure may be Al—Si—Fe alloys. FIG. 15 presents a phase diagram for a near-eutectic Al—Si—Fe alloy. In some embodiments, the atomic % of Al in the Al—Si—Fe alloy of the present disclosure can vary between about 40 at. % to about 50 at. %. In some embodiments, the atomic % of Si in the Al—Si—Fe alloy of the present disclosure can vary between about 35 at. %. to about 45 at. %. In some embodiments, the atomic % of Fe in the Al—Si—Fe alloy of the present disclosure can vary between about 10 at. %. to about 20 at. %. In some embodiments, the atomic ratio between the elements of the Al—Si—Fe alloy is 45:40:15, i.e. the alloy is 45Al-40Si-15Fe.

In reference to FIG. 16, the PCMs of the present disclosure may be incorporated into thermal energy storage modules 1600 for storage of thermal energy in various energy storage applications. In some embodiments, thermal energy storage modules 1600 may include PCMs configured to have different melting temperatures.

FIG. 16 presents a schematic diagram of using the thermal storage modules 1600 of the present disclosure for thermal energy storage and release. For example, in the charging phase, heat may be transferred into the PCMs inside the thermal storage modules 1600 to charge the PCMs. For example, during the charging phase, heat from an outgoing process fluid at a high temperature may be transferred into the PCMs to cool the process fluid down for treatment or storage. When a cooled process fluid needs to be introduced back to the process, heat stored in the PCMs of the thermal storage modules 1600 may be allowed to transfer to the incoming process fluid to bring the process fluid to the temperature of the process, thereby discharging the PCMs. In some embodiments, the thermal storage modules 1600 of the present disclosure can be used for energy storage in concentrated solar-thermal power applications. For example, thermal storage modules 1600 of the present disclosure can be combined with concentrated solar-thermal power (CSP) system to store heat produced during the daytime (charging phase) and release overnight the heat saved during daytime (discharging phase) to generate electricity by thermoelectric module system. In some embodiments, thermal storage modules 1600 of the present disclosure can be used for TES to collect and save the exhaust heat from nuclear power station. In some embodiments, thermal storage modules 1600 of the present disclosure may be at least partially encapsulated in a thermal insulator material to prevent undesired heat loss from the PCMs.

To prepare the PCMs of the present disclosure, first powders of constituents of the base alloy and one or more additional elements in desired concentrations can be uniformly mixed together, and then the mixture can be consolidated under pressure and elevated temperature. In some embodiments, the mixture of powders may be consolidated by hot pressing. In some embodiments, direct current induced hot press can be used. Other consolidation techniques known in the art may also be used with the presently disclosed embodiments. By way of a non-limiting example, FIG. 17A and FIG. 17B present an SEM image of Al-12 Si alloy prepared by hot press method, using 80 mPa of pressure at temperature of 577° C., and results of latent heat test of such alloy, respectively. The disc had a diameter of 12.7 mm and thickness of between 1.5 and 2.8 mm. Another example is presented in FIG. 18A, FIG. 18B, FIG. 18C and FIG. 18D, which present SEM images of a disc prepared from Al-20Si (by weight percent). The disc was prepared at 80 mPa of pressure and at 577° C., and had a diameter of about 1.27 cm.

The methods and materials of the present disclosure are described in the following Examples, which are set forth to aid in the understanding of the disclosure, and should not be construed to limit in any way the scope of the disclosure as defined in the claims which follow thereafter. The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the embodiments of the present disclosure, and are not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all or the only experiments performed. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental errors and deviations should be accounted for.

EXAMPLES Example 1 Synthesis of Ternary Near-Eutectic Al—Si—Fe Alloys

From the ternary phase diagram of Al—Si—Fe system, the chemical composition of the near-eutectic point is around 45Al-40Si-15Fe (at %). In order to synthesize the bulk near-eutectic alloys for the measurement of latent heat and other characteristics, Al, Si, and Fe powders purchased from Alfa Aesar were uniformly mixed, followed by hot pressing at different temperatures (300, 500, 700, and 1000° C.), with a starting pressure at 94-110 MPa before heating (at room temperature).

FIG. 19A, FIG. 19B, and FIG. 19C are SEM images of Al powder (grain diameter ˜20-40 μm), Si powder (grain diameter: nanoscaled), and Fe powder (grain diameter ˜2-4 μm), respectively. FIG. 19D is an SEM image of the synthesized Al—Si—Fe near-eutectic alloy using DC hot-press method.

Based on the melting point at around 875° C. for the near-eutectic Al—Si—Fe alloy, the hot-press temperature of 1000° C. was firstly tried (see FIG. 19D). However, it was noted that the original Al and Si powders should be melted preferentially at around 577° C. due to the formation of eutectic Al—Si alloy. In this case, some melting component (in liquid) might be squeezed out of the graphite die before the formation of ternary near-eutectic alloy (Al—Si—Fe), which might result in deviation of chemical composition out of the designed chemical composition (at %, 45Al-40Si-15Fe).

In order to decrease or prevent the loss of Al and Si during the hot-press, the eutectic alloy powder was pressed at 700° C. at lower static pressure (at 94 MPa, rather than 110 MPa at 1000° C.). FIG. 20A and FIG. 20B, which is a close up of FIG. 20A, show the fractured surface of the ternary near-eutectic sample hot pressed at 700° C. Since the hot-press temperature 700° C. is still higher than the melting point of eutectic Al—Si alloy (577° C.), there are still some liquid Al—Si alloy squeezed out of the graphite die during the hot-press operation. It is also clear that some eutectic Al—Si alloy particles have formed shown by the crystals in the middle of the image of FIG. 20B.

FIG. 20C and FIG. 20D, which is a close up of FIG. 20C, show the fractured surface of the ternary near-eutectic sample hot pressed at 500° C. With these hot-press conditions, there is no melting occurred, no loss of powder during the hot press. However, the sample shows that the near-eutectic alloy did not form, as shown in FIG. 20D. Using this hot-pressed bulk sample, DSC test was carried out to measure the latent heat of fusion of near-eutectic ternay Al—Si—Fe alloy.

Example 2 Experimental DSC Data for the Synthesized Ternary Near-Eutectic Al—Si—Fe Alloy

Considering the melting point of the near-eutectic Al—Si—Fe alloy is around 875° C. (by phase diagram), the highest temperature was set to 1000° C. for the DSC experiments. FIG. 21A presents DSC experimental results of the ternary near-eutectic Al—Si—Fe alloy hot-pressed at 1000° C. (with applied pressure around 94 MPa). FIG. 21B presents DSC experimental results of the ternary near-eutectic Al—Si—Fe alloy hot-pressed at 700° C. (with applied pressure around 94 MPa). FIG. 21C presents DSC experimental results of the ternary near-eutectic Al—Si—Fe alloy hot-pressed at 500° C. (with applied pressure around 94 MPa).

FIG. 21A and FIG. 21B (hot-pressed at 1000° C.) and FIG. 21C and FIG. 21D (hot-pressed at 700° C.) show that the melting point is consistent with the phase diagram close to 875° C. and the latent heat is ˜865 kJ/kg that is much higher than the proposed value of 500 kJ/kg. This data shows that there is little difference between these two different hot-press operations.

FIG. 21E and FIG. 21F look considerably different from FIGS. 21A-21B and FIGS. 21C-21D, since the obtained latent heat (610.6 kJ/kg) is much less than those shown in FIGS. 21A-21B and FIGS. 21C-21D. The data in FIG. 21E and FIG. 21F are not correct due to the following two reasons: (1) as shown in FIG. 21E and FIG. 21F, the original Al/Fe/Si powder was physically mixed and hot-pressed at 500° C., no melting occurred during the press; (2) after the DSC measurement, it is noted that the sample splitted into two parts, one is round alloy (melted), and another is composed of black powder (maybe Fe/Si powder), which means the remained Si and Fe particles did not take part in the reaction of near-eutectic alloying during the DSC testing/heating. Therefore, the near-eutectic alloy needs to be made before any meaningful measurements can be obtained.

On the other hand, based on the previous experimental DSC data, the latent heat of this near-eutectic Al—Si—Fe alloy could be estimated by a simple addition of the latent heat of each pure component/element multiplying the corresponding percentage. For the ternary near-eutectic Al—Si—Fe alloy, the estimated latent heat could be simply calculated as the following:

Al(38.24 wt %)—latent heat: 397×0.3824=151.81 kJ/kg

Si(35.38 wt %)—latent heat: 1800×0.3538=636.84 kJ/kg

Fe(26.38 wt %)—latent heat: 247×0.2638=65.16 kJ/kg

Therefore, the total latent heat might be close to 853.8 kJ/kg, which is reasonable and consistent with the experimental data, as shown in FIG. 21A and FIG. 21B.

Example 3 Theoretical Calculations

At constant pressure, phase diagram of alloys can be determined by finding the minimum Gibbs free energy among all the phases at different temperature. When different species are mixed together, the change of Gibbs free energy (G) from mixing can be expressed as,

ΔG=ΔH−TΔS

where H is enthalpy, T is temperature and S is entropy.

The enthalpy change ΔH can be either negative or positive depending on the atomic bonding strength. When the bonding strength between atoms with different types is stronger than that between the same types of atoms, the mixing enthalpy is negative, and energy needs to be provided to form the mixture (or alloy). If the bonding strength in the mixture (or alloy) is week, the mixing enthalpy is positive. FIG. 22 illustrates the formation of phase diagram with a eutectic point. At high temperature T₁, the Gibbs free energy of liquid phase is always lower, and only liquid phase exist. With decreasing temperature (T₂, T₃), two-phase regions shows up when there are common tangents to both the liquid and solid curve. When there is only one common tangent to the liquid and solid curve (T₄), it reaches the eutectic point. With temperature lower than eutectic temperature (T₅), only solid phases exist.

When the mixing enthalpy is positive, the Gibbs free energy curve would show a convex shape in the middle composition as shown in FIG. 22 for the solid phase (with symbol S). When the mixing enthalpy is negative, the Gibbs free energy always shows a concave shape as shown in FIG. 22 for the liquid phase (with symbol L). With positive mixing enthalpy for the solid phase and negative mixing enthalpy for the liquid phase (or the bond is weak in the solid and strong in the liquid), there is a common tangent of the solid and liquid Gibbs free energy curve at certain temperature where the eutectic alloy forms. This temperature is called the eutectic temperature, which is always lower than the melting temperature of the elements as shown from FIG. 22. However, if the mixing entropy is negative for the solid and positive for the liquid (or the bond is strong in the solid and weak in the liquid), it could form an intermediate phase with melting temperature higher than any of the elements. The above analysis can be used to guide material selections.

Experimental measurements show that the latent heat of fusion of Cu-16Si is less than 183.5 kJ/kg with eutectic temperature at 806.9° C. One reason for the low latent heat of fusion is because Si exists mainly in the form of SiCu₃ compound after phase separation as shown in FIG. 23, which illustrates a phase diagram for an Cu—Si alloy from ASM Alloy Phase Diagram Center. The large latent heat of fusion of Si does not contribute to that of the eutectic alloy. Another reason is the SiCu₃ compound has a relatively low melting temperature, which means the atomic bond of Si—Cu is relatively week in solid phase and relatively strong in liquid phase, so that the SiCu₃ compound has a low latent heat of fusion (per mass) which is even smaller than Cu.

From the above analysis, alloys with the following properties can be selected for high latent heat of fusion: (1) materials with high ratio of latent heat over melting temperature,

$\frac{\Delta \; H_{f}}{T_{m}}$

(such as Si, Al, B, etc.) can exist as much as possible in their pure element form after solidification; (2) the formed intermediate compound should have a high melting temperature; (3) the elements should have relatively even composition in order to increase the mixing entropy; (4) light elements should be used.

Based on the above rule, the Pd—Si eutectic alloy with 52% (at. %) Si could be a good choice as shown in FIG. 24, which illustrates a phase diagram for a Pd—Si alloy from ASM Alloy Phase Diagram Center. After phase separation, there is a large amount of Si in its single element form which can contribute significantly to the latent heat of fusion of the alloy. The intermediate compound Pd₂Si has a high melting temperature which means the atomic bond of Pd₂Si is strong in solid phase and weak in liquid phase, so that Pd₂Si could have a large latent heat of fusion, which can also contribute to the latent heat of fusion of the eutectic alloy. However, Pd may be too expensive for large-scale applications.

Another alloy that was analyzed is the ternary alloy Al—Fe—Si with phase diagram shown in FIG. 25, which illustrates a phase diagram for a Al—Fe—Si alloy from ASM Alloy Phase Diagram Center. This alloy has a narrow melting temperature range around 890° C. with composition of 40% Si, 45% Al and 15% Fe in atomic percentage. The compositions of the three elements are relatively even, and the entropy from mixing can contribute significantly to the latent heat of fusion. There is also significant amount of Si which can greatly increase the latent heat. Measurement shows the latent heat of fusion is about 865 kJ/kg with a peak melting temperature around 874.9° C., which is significantly larger than Cu-16Si alloy.

In some embodiments, a thermal energy storage module includes one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg. In some embodiments, the one or more phase change alloys have a formula of Aluminum (Al)-Silicon (Si)—X, wherein X is one or more elements selected from the group consisting of boron (B), carbon (C), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), germanium (Ge), molybdenum (Mo), nobium (Nb), nickel (Ni), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V), and zirconium (Zr). In some embodiments, the one or more phase change alloys have a formula of Al—Si—Fe, where the concentration of Al is between about 40 atom percent and about 50 atom percent, the concentration of Si is between about 35 atom percent and about 45 atom percent and the concentration of Fe is between about 10 atom percent and about 20 atom percent. In some embodiments, a phase change alloy has a formula Al—Si—Fe, where the concentration of Al is about 45 atom percent, the concentration of Si is about 40 atom percent, and the concentration of Fe is about 15 atom percent. In some embodiments, a phase change alloy is Al—Si—B, where the concentration of B is between about 8 atom percent and about 28 atom percent.

In some embodiments, a method of storing heat includes providing one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg; charging the one or more phase change alloys by allowing heat transfer into the phase change alloy; and storing heat in the one or more phase change alloys.

All patents, patent applications, and published references cited herein are hereby incorporated by reference in their entirety. While the methods of the present disclosure have been described in connection with the specific embodiments thereof, it will be understood that it is capable of further modification. Furthermore, this application is intended to cover any variations, uses, or adaptations of the methods of the present disclosure, including such departures from the present disclosure as come within known or customary practice in the art to which the methods of the present disclosure pertain. 

What is claimed is:
 1. A thermal energy storage module comprising one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg.
 2. The thermal energy storage module of claim 1, wherein the one or more phase change alloys have a formula of Aluminum (Al)-Silicon (Si)—X, wherein X is one or more elements selected from the group consisting of boron (B), carbon (C), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), germanium (Ge), molybdenum (Mo), nobium (Nb), nickel (Ni), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V), and zirconium (Zr)
 3. The thermal energy storage module of claim 2, wherein X is Fe.
 4. The thermal energy storage module of claim 3, wherein the concentration of Al is between about 40 atom percent and about 50 atom percent.
 5. The thermal energy storage module of claim 3, wherein the concentration of Si is between about 35 atom percent and about 45 atom percent.
 6. The thermal energy storage module of claim 3, wherein the concentration of Fe is between about 10 atom percent and about 20 atom percent.
 7. The thermal energy storage module of claim 3, wherein the concentration of Al is about 45 atom percent, the concentration of Si is about 40 atom percent, and the concentration of Fe is about 15 atom percent.
 8. The thermal energy storage module of claim 2, wherein X is B in concentration of between about 8 atom percent and about 28 atom percent.
 9. A thermal energy storage module comprising one or more phase change alloys having a general formula of Al—Si—Fe, wherein the concentration of Al is between about 40 atom percent and about 50 atom percent, the concentration of Si is between about 35 atom percent and about 45 atom percent, and the concentration of Fe is between about 10 atom percent and about 20 atom percent.
 10. A method of storing heat comprising: providing one or more phase change alloys having a variable phase transition temperature between about 400° C. and about 1200° C. and having a latent heat of more than about 200 kJ/kg; charging the one or more phase change alloys by allowing heat transfer into the phase change alloy; and storing heat in the one or more phase change alloys.
 11. The method of claim 10, wherein the one or more phase change alloys have a formula of Aluminum (Al)-Silicon (Si)—X, wherein X is one or more elements selected from the group consisting of boron (B), carbon (C), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), germanium (Ge), molybdenum (Mo), nobium (Nb), nickel (Ni), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V), and zirconium (Zr)
 12. The method of claim 11, wherein X is Fe.
 13. The method of claim 12, wherein the concentration of Al is between about 40 atom percent and about 50 atom percent.
 14. The method of claim 12, wherein the concentration of Si is between about 35 atom percent and about 45 atom percent.
 15. The method of claim 12, wherein the concentration of Fe is between about 10 atom percent and about 20 atom percent.
 16. The method of claim 12, wherein the concentration of Al is about 45 atom percent, the concentration of Si is about 40 atom percent, and the concentration of Fe is about 15 atom percent.
 17. The method of claim 11, wherein X is B having the concentration between about 8 atom percent and about 28 atom percent.
 18. The method of claim 10, wherein, in the step of charging, heat is produced by a solar-thermal power system.
 19. The method of claim 18 further comprising releasing heat stored in the one or more phase change alloys to generate electricity.
 20. The method of claim 10, wherein, in the step of charging, heat is an exhaust heat from a nuclear power station. 